Re: [PATCH v2] rcu: Reduce synchronize_rcu() delays when all wait heads are in use

[Paul E. McKenney]

On Tue, Apr 09, 2024 at 10:49:30AM +0200, Uladzislau Rezki wrote:
> Hello, Neeraj, Frederic!
> 
> > 
> > On 4/5/2024 3:12 AM, Frederic Weisbecker wrote:
> > > Le Wed, Apr 03, 2024 at 04:22:12PM +0530, Neeraj Upadhyay a écrit :
> > >> When all wait heads are in use, which can happen when
> > >> rcu_sr_normal_gp_cleanup_work()'s callback processing
> > >> is slow, any new synchronize_rcu() user's rcu_synchronize
> > >> node's processing is deferred to future GP periods. This
> > >> can result in long list of synchronize_rcu() invocations
> > >> waiting for full grace period processing, which can delay
> > >> freeing of memory. Mitigate this problem by using first
> > >> node in the list as wait tail when all wait heads are in use.
> > >> While methods to speed up callback processing would be needed
> > >> to recover from this situation, allowing new nodes to complete
> > >> their grace period can help prevent delays due to a fixed
> > >> number of wait head nodes.
> > >>
> > >> Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx>
> > > 
> > > Looking at it again, I'm not sure if it's a good idea to
> > > optimize the thing that far. It's already a tricky state machine
> > > to review and the workqueue has SR_NORMAL_GP_WAIT_HEAD_MAX - 1 = 4
> > > grace periods worth of time to execute. Such a tense situation may
> > > happen of course but, should we really work around that?
> > > 
> > > I let you guys judge. In the meantime, I haven't found correctness
> > 
> > I agree that this adds more complexity for handling a scenario
> > which is not expected to happen often. Also, this does not help
> > much to recover from the situation, as most of the callbacks are still
> > blocked on kworker execution. Intent was to keep the patch ready, in
> > case we see fixed SR_NORMAL_GP_WAIT_HEAD_MAX  as a blocking factor.
> > It's fine from my side if we want to hold off this one. Uladzislau
> > what do you think?
> > 
> I agree with Frederic and we discussed this patch with Neeraj! I think
> the state machine is a bit complex as of now. Let's hold off it so far.

There is always the next merge window, should it be required.

							Thanx, Paul